Sorry for the simple question but where can I display my serial number in the zbrush interface? I can’t find it anywhere.
You can’t. This is for security reasons, such as if a school installs ZBrush they wouldn’t want the students to be able to pull up the serial number and use it for themselves.
If you have forgotten your serial number, check your emails. It will be in the delivery email sent by Cleverbridge after your purchase of (or upgrade to) ZBrush 4. If you no longer have that email, open a Support ticket so that we can resend it to you. Be sure to include the name and email address that would be on the account so that we can find you.